This invention is concerned with pressure-operated linear actuators in which the working liquid is an Electro-Rheological (ER) fluid.
Electro-rheological (ER) fluids are slurries of finely-divided solids in base liquids. Their flow behaviour is normally approximately Newtonian, like that of pure liquids, but exposure to an electric field evokes a large increase in flow resistance; this change is progressive (i.e. the greater the field, the greater the increase in flow resistance), reversible and occurs virtually instantaneously.
If an ER fluid is pumped through an array of insulated fixed plates with an electric field between them, the pressure drop across the assembly increases with the field; the system behaves like a servo-valve and is described as an "ER Valve". ER valves can take many forms:--they can be made up of flat plates, or concentric tubes and rods inside tubes. Having no moving parts, ER valves are cheap to make and their speed of response is very much faster than a conventional electromagnetic servo-valve. On the other hand, the electrical power supply to ER valves must be at high voltage, typically 2-4 kV, depending on the gap between the electrodes. The cost of high voltage units increases very sharply with the power required, and if their current output exceeds about 10 mA, they are potentially lethal. It is therefore important to minimise the power requirement of ER valves in any given application.
A complete, functional actuator system will include several items in addition to the ER fluid and the actuator. A pump will be required to generate the flow and pressure required to operate the device. A source of electrical power at high voltage will be required to energise the ER valve and this will need a control system to generate the command signals, and, in the case of a vibrator, to prevent the actuator "drifting" to one end of its permitted travel. All these are already available, or can be derived fairly simply from corresponding elements in conventional technology. They form no part of the present Patent, which is concerned solely with the actuator device itself.
Actuators combining ER Valves with piston assemblies similar to those used in conventional hydraulics are well known. Equations are available which adequately describe the pressure and flow of a known ER fluid through an ER valve. By combining these with the calculation techniques developed for conventional hydraulic devices it is possible to specify such actuators in terms of the properties of the ER fluid, the supply pressure, the piston area and the length, width and gap of the ER valve so that the peak power output occurs at any desired thrust and stroking speed. These calculations must also include the electrical control power required, since the high voltage source is usually the most costly single component in the system. The conventional optimisation methods must be slightly modified to minimise the control power required to achieve the desired performance. These methods and the practical results thereof are well known to those skilled in the art and form no part of the present Patent. However, in actuators designed for fast response, an additional parameter must be included in the calculations. This invention is concerned with constructional modifications imposed by this additional parameter and by the nature of ER fluids themselves.
In electro-magnetic servo-valves, the maximum band-width of 100-150 Hz is achieved by making the moving parts small and light and minimising their travel. The resultant restriction of the flow path through an "open" valve limits the maximum piston speed. By contrast, an ER valve has no moving parts and the flow path is limited only by the space and the electrical control power available, the latter being determined by the gap between the electrodes and their area. The ratio of the length of the electrode plates to the gap between them is fixed by the required maximum operating pressure; their width, and the gap between them determine the no-field flow resistance which can therefore be reduced as far as desired, but at the cost of increasing the electrical control power. This feature, combined with the very fast response of suitable ER fluids to changes in the electric field, which has a band-width of about 1 kHz, make it possible for ER actuators and vibrators to work at much higher frequencies and piston speeds than conventional systems, but aspects which can usually be neglected in conventional hydraulic systems become so important at high frequencies that they determine the basic design.
"Referred Inertia", i.e. the anomalously high inertial effects of small amounts of fluid in long, narrow pipes. is well known in conventional hydraulics, but it is seldom important in actuators. These usually work at relatively high pressures and are therefore fairly small and compact and as already discussed, their high frequency performance is limited by other considerations. The pressure capability of an ER valve, which is equivalent to a pipe, is directly proportional to its length, so long "pipe" runs are unavoidable. Furthermore, the specific gravity of a typical ER fluid (1.4) is much greater than that of a hydrocarbon oil (0.8-0.9). Detailed calculations show that referred inertia becomes the dominant term at high frequencies and amplitudes, so high frequency ER actuators must be designed to minimise this. In practice, since the size and shaped of the ER valves are fixed by other considerations, this means that the interconnecting pipes must be as short and wide as possible.
A second important design requirement arises from the fact that ER fluids are suspensions of fine particles in a base liquid. Although the solid and liquid in modem ER fluids are selected to have the same density, this "density matching" can only be exact at one temperature, so some settling-out is unavoidable. The practical disadvantages of this can be minimised by designing the actuator so that, as far as possible, the flow is uni-directional in every part; if "tidal flow" is allowed to occur at any point, for example into and out of a piston and cylinder assembly, a small mis-match in density between liquid and solid will eventually lead to the latter accumulating in the closed end. If there is a "through flow", on the other hand, any accumulated solid is swept out and re-dispersed.